Purification of naphthenic acids



Dec. 25, 1945. w. E. MOCORQUODALE, JR 2,391,729

PURIFICATION OF NAPHTHENIC ACIDS Filed Oct. 14, 1942 2 Sheets-Sheet 2 30L VENT A A V 1w ZVVW' A ,WV\%

IVAPHTHEN/C M may; OIL

ACIDS /VA Pl/THL-W/C /00% 0/4 #6105 3 I Wye/W0? MMESS" I wrf/v lyzzaa/a/qfi w 5y Patented Dec. 25, 1945 PURIFICATION OF NAPHTHENIC ACIDS Wilmer E. McCorquodale, Jr., Ardentown, Del.,

assignor -to Sun Oil Company, Philadelphia Pa., a corporation oi New Jersey Application October 14, i942, Serial No. 462,035l 7 Claims. Cl. 260-514) The present invention relates to a method of obtaining naphthenic or petroleum acids of any desired degree of purlty'from mixtures of such naphthenic acids and neutral hydrocarbon oils.

' The organic acids derived from petroleum" have found wide and varied'uses in numerous arts. These acids will be referred to herein as naphthenic acids and it is to be understood that this term is used herein in a broadsense and includes;-

not only the true naphthenic acids of the series of CnHzn-cO: but also acids of other series such.

as CuHZn-z02 in which a: may vary from '2 to 8 or higher, and which are derived from petroleum, all of which are commonly referred to as petroleum naphthenic acids. However, such naphthenic acids are generally present in relatively small amounts in any given crude petroleum and it is 'dilficult to obtain them in any relatively pure form. While for many uses, for instance in the manufacture of emulsifiable oils, the acids or more particularly their soaps are used in admixture with neutral hydrocarbon oils and it is therefore not necessary to purify the acids ex-.

tensively, for many of the more recently developed uses, for instance as plasticizers in rubber manufacture, naphthenic acidswhich aresubstantially free of neutral oils must be obtained and there is at present a considerable market for pure naphthenic acids; andfor many. of the uses to which these acids are now' put naphthenic acids containing two per cent or less of neutral oils are specified.

In order to produce acids of this degree of purity by known processes, the acids are generally converted into their corresponding soaps and the soaps are then separated from the oil by solvent extraction or distillation or some combination of distillation and solvent extraction. The soaps must then be acidified to convert them back into their corresponding naphthenic 'acids and the naphthenic-acids thus obtained then desalted. Such purification methods require' relatively large amounts of chemicals (alkali andv mineral acid) which are generally lost as salt, so that the chemical costs of such processes are quite substantial.

Eflorts have been made to separate the naphthenic acids from neutral hydrocarbon oils by straight solvent extraction without saponiflcae tion of the acids. However, such straight solvent extraction processes as have been suggested involve the use of solvents .with which mixtures of naphthenic acids and oil containing less than a certain amount of oil are miscible in all proportions and therefore by known solvent extraction methods it is impossible to obtain naphthenic acids of a relatively high degree of. purity.

The present invention is therefore directed to a method of straight solvent extraction of:naphthenic acids, without saponification of-the: acids,

in which the solvent employed is one which while it has preferential solvency for naphthenic acids is not completely miscible with naphthenicacids. and in which the extraction conditions employed are such that naphthenic acids of any desired degree of purity may be obtained.

In order to more readily describe the present invention there is attached hereto drawings in which:

Fig. 1 is a diagrammatic flow chart of apparatus suitable for practicing the present invention. I

Fig. 2 is a phase diagram of the system, naphthenic acidsoil and solvent in which thesolvent employed is miscible with the naphthenic acids in all proportions, while Fig. 3 is a phase diagram of the system, naphthenic acidsoil and solvent in which the solvent employed is not completely miscible with naphthenic acids.

Among the solvents which have been suggested for purification of naphthenic acids are methanol and glacial acetic acid. However, when these solvents are employed a phase diagram of the type illustrated in Fig. 2 is obtained and naphthenic acids of a greater purity than M cannot be obtained, no matter what extraction conditions are employed. I have discovered that if any polar organic liquid having preferential solvency for naphthenic acids but which is not completely miscible therewith is employed, rather than a completely miscible solvent such as those heretofore suggested, a system of the type illustrated in F 3 may be obtained and, by the use of proper extraction conditions; i. e. by using sumcient reflux and the proper number of extraction stages, naphthenic acids of any desired degree of purity may be-obtained. In practicing the present invention I may employ either a single solvent.

which has selective preferential solvency for naphthenic acids and which is-not completely miscible with naphthenic acids; or I may employ one of the solvents already known to have preferential solvency for naphthenic acids but which is completely miscible, and add to such solvent a second liquid which causes its miscibility'with naphthenic acids to decrease." Among the solvents which I have found to be suitable are: methanol containing no less than 12 vol. per cent water:

methanol containing no less than 26.5 vol. per

cent of commercial formic acid (or including the water, the actual composition of this solvent is 73.5 per cent methanol, 22.7 per cent anhydrous formic acid-and 3.8 per cent water) a third suitable solvent is acetic acid containing no less than 5.7 vol. per cent water. By commingling the crude acids with a solvent of the type specified above, and employing suitable extraction conditions I can obtain naphthenic acids of any desired degree of purity. Preferably I employ countercurrent extraction with return of a portion of the extract to the extraction zone as reflux, and a preferred method of practicing my invention will now be described, reference being made to Fig. l of the accompanying drawings.

The crude naphthenic acids which it is desired to purify and which may comprise a mixture of naphthenic acids and neutral oil are withdrawn from tank I by pump 2 and pumped through line 3-to mixer 4 wherein they are mixed with the amount of solvent introduced through line I required to give a substantially saturated mixture which will not separate into two phases. The mixed crude naphthenic acids and solvent are then introduced into extraction column 5' at a point near the center and therein they are commingled with a relatively large amount of solvent. In the column 5 there flows downwardly an oil layer countercurrent to an upwardlyflowing solvent layer that is in equilibrium therewith. At the top of the extraction column 5 a saturated solvent phase is removed through line 6 while from the bottom of the column 5 a saturated raflinate product is removed through line I. The saturated solvent phase removed from the top of column 5 through line 6 is introduced into evaporator I wherein a suflicient quantity of solvent is distilled oil, without changing the composition of the solvent, so as to yield a bottom product with a composition corresponding to a saturated extract phase.

This removal of solvent may be accomplished in any desired manner and is only diagrammatically illustrated in the drawings, the saturated solvent phase introduced into evaporator I is heated-by steam coil 8 therein in order to vaporize the required proportion of the solvent and leave a saturated extract phase which is withdrawn from the bottom of evaporator i through line 9. A portion of the saturated extract phase flowing through line 9 is introduced through line ill into still Ii wherein it is heated by steam coil i2, the solvent remaining therein being vaporized and removed overhead through line I3 to line I5, and flowin therethrough to condenser l6 wherein it is condensed and from which it flows through line H to solvent storage tank i8. From still H the reflned naphthenic acids, free of solvent, are withdrawn through line l9 and flow to storage tank 20. A portion of the saturated extract phase withdrawn from evaporator I through line 9 is passed through line 21 to cooler 22 wherein it is cooled by indirect heat exchange to the temperature employed in extraction column 5 and after cooling is introduced through line 23 into the top of extraction column 5 to serve as reflux. The reflux ratio employed is that necessary to give a saturated solvent phase at the top of column 5 in which the naphthenic acids will be of the desired degree of purity.

The saturated raflinate product withdrawn from bottom of extraction column 5 through line 1 flows therethrough to heater 25 wherein it is" heated to a suflicient temperature to vaporize the solvent on reduction of pressure and flows thence through line 26 to flash tower 21 wherein the pressure is reduced and the solvent vapors formed are separated from the raflinate, the vapors being taken overhead through line 28, flowing therethrough to line i5 and thence to condenser i8 and solvent storage tank 18. The railinate is removed from the bottom of flash tower 21 through line 29 and flows therethrough to raffinate storage tank 30. The remainder of the saturated raflinate product removed from the bottom of extraction column 5 through line I flows through line 3i to mixer 32 wherein it is mixed with solvent withdrawn from solvent storage tank It through line 33 and the mixture introduced as reflux through line 34 into the bottom of extraction column 5. A sufiicient amount of saturated raflinate product is returned as reflux into the bottom of extraction column 5 in admixture with solvent from line 33 to give a saturated solvent phase which is substantially in equilibrium with the saturated raflinate product withdrawn from the bottom of the column. A sufllcient quantity or'solvent is introduced into the bottom of extraction column 5 through line 33 in order to effect the desired separation or degree of purity. v Thequantity of solvent is dependent on the reflux ratio that must be employed to make this separation. This reflux ratio will oi course vary with the height of column. 1. e. the number of theoretical plates 'or extraction stages, the composition of the feed, and the solubility relationships of the naphthenicacids, neutral hydrocarbon oil, and solvent.

For any given operation of the column there are actually two reflux ratios employed, one at the top of the column and the other at the bottom, and these are not necessarily the same. However, for any specified degree of separation of the feed components the two are related; thus for any given amount of reflux at one end of the column the reflux for the other end is flxed. Therefore,

in specifying the column operation it is necessary to specify the reflux ratio at only one end of the column. Suitable mathematical and graphical methods of determining the necessary reflux are described by Varteressian and Fenske in Industrial and Engineering Chemistry, volume 29, Number 3, pages 270-277, March 1937, and need not be further described here.

However, in order to fully describe the present invention the following specific example is given. The crude naphthenic acids to be purified were obtained by the method disclosed in Terrell, Hughes and Carter Patent No. 2,056,913 and consisted of 75 per cent naphthenic acids and 25 per cent neutral oils by weight. It was desired to separate these acids into an extract comprising 98 per cent naphthenic acids and 2 per cent neutral oils. In order to efiect the separation the crude acids were mixed with about 20 per cent by weight of a solvent comprising 73.5 per cent methanol and 26.5 per cent commercial formic acid by volume. The mixture which was' obtained was substantially saturated and did not separate into two phases. This mixture was introduced into an extraction column at a point near the center thereof and was therein commingled with a large quantity of solvent. The saturated solvent phase was withdrawn from the top of column 5 through line 8 and this saturated solvent phase consisted of 80.6 per cent solvent, 19 per cent naphthenic acids and 0.4 per cent of neutral oil. The greater part of the solvent was removed from' the saturated solvent phase by distillation in evaporator 'I so as to yield a saturated extract product having a composition of 60.5 per cent naphthenic acids, 38.2 per cent solvent and 1.2 per cent oil. A reflux ratio of 1.3 to 1 was employed at the top of the columns and with this reflux ratio an extraction column having a height equivalent to six theoretical plates was required in order to eilect thedesired purification. Thus 1.3 parts of the saturated extract product withdrawn from evaporator 7 through line 3 were returned through lines ii and 23 as reflux to column to each part of saturated extract product withdrawn from the system through line It. The

saturated extract product withdrawn from the system through line It, flowed to still H, wherein the remainder of the solvent was evaporated on to yield an extract consisting of 98 per cent naphthenic acids and 2 per cent neutral oil. From the bottom of extraction column 5 a saturated ramnate product comprising '73 per cent oil and 19 per cent naphthenic acids and 8 per cent solvent was withdrawn, and a sufilcient amount of solvent to effect the desired separation was introduced at this point through line 34, such-a portion of the saturated rafi'inate product flowing through line I being mixed therewith in mixer 32 as was required to give a solvent phase entering the extraction column which was in substantial equilibrium with the saturated rafiinate product withdrawn therefrom.

In operating in the above manner, the exact amounts of the materials fed to the process and the products obtained for each hundred pounds of commercial naphthenic acids processed were as follows: One hundred pounds of commercial naphthenic acids which were composed of 74.1 pounds naphthenic acids and 25.9 pounds neutral oil were withdrawn from tank I and introduced through line 3 to mixer! wherein they were mixed with 23.2 pounds of the above identified solvent and the mixture then introduced into extraction column 5. At the bottom of column 5 586.1 pounds of solventwere introduced through line 34 from mixer 32 and line 33. At the top oi column 5 through line 6 a saturated solvent phase composed of 662.5 pounds of solvent, 156.3 pounds of acids and 32 pounds of oil were withdrawn. In evaporator I 563.5 pounds of solvent were evaporated to yield a saturated extract product which was withdrawn from evaporator I through line 9. A reflux ratio of 1.3:1 was employed at the top of extraction column 5 so the saturated extract product flowing through line 9 3 amount of solvent to the crude acids fed to the process to saturate the feed, it is of course possible to introduce the crude acids directly into the extraction column and by using a higher column saturate the feed in the column. It should also be understood that other extraction methods may be employed, although the method specifically described herein is preferred.

- I claim:

1. In the purification of naphthenic acids by solvent extraction, th method which comprises feeding crude naphthenic acid containing hydrocarbon oil to a countercurrent extraction zone between its ends, feeding into one end of said zone a solvent having preferential solvency for and incomplete miscibility with-the naph 'thenic acids and comprising at least a, major percentage of polar organic liquid, the solvent flowing through said zone countercurrently' to the feed, withdrawing from the other end of the zone a saturated solvent phase and from the solvent-feed end of the zone a saturated rafiinate phase, removing solvent from the saturated solvent phase to thereby separate a naphthenic g acids-rich extract, returning as reflux to the solvent phase-withdrawal end of the extraction zone a part of said extract together with a minor proportion of solvent,and removing solvent from the rafiinate phase to yield an oil-rich raflinate. 1

2. The method defined in claim 1 wherein the solvent comprises a polar organic liquid contain- .ing a minor proportion of water.

.3. The method defined in claim l wherein the solvent comprises methanol containing not less F than 12 volume per cent water.

4. The method defined in claim 1' wherein the solvent comprises a major percentage of methanol and a minor water.

5. The method defined in claim 1 wherein the percentage of formic acid and solvent comprises acetic acid containing not less than 5.7 volume per cent water.

6. The method defined in claim 1 wherein the said feed is mixed, prior to its introduction into the extraction zone, with such minor proportion of said solvent that the mixture fed to said zone is substantially saturated with said solvent.

7. In the purification of naphthenic acids by solvent extraction, the method which comprises feeding crude naphthenic acids containing hydrocarbon oil to a countercurrent'extractionzon between its ends, feeding into one end of was split into two streams one of which amounting to 43 pounds of solvent, 68 pounds of acids and 1.39 pounds of oil was withdrawn from the It should be understood that the foregoing is a detailed description of a preferred method of practicing the present invention and; that various modifications and changes may be made therein without departing from the spirit of this invention or the scope of the appended claims.

Thus. while it is preferre to add a sumcient for and incomplete 23tothetopofcolumn5' said zone a solvent having preferential solvency miscibility with the naphthenic acids and comprisin at least a major percentage of polar organic liquid. the solvent flowing through said zone countercurrently to the feed, withdrawing from the other end of the zone a saturated solvent phase and from the solvent-feed end of the zone a saturated ramnate phase, removing solvent from the saturated solvent phase to thereby separate a naphthenic acids-rich extract, returning as reflux to the solvent phase-withdrawal end of the extraction zone a part of said extract together with a minor proportion of solvent. returning a portion of the raiiinate phase to the extraction zone along with fresh solvent .and removing solvent from the remainder of the raiilnate phase to yield an oilrich raflinate. Y I

I WILMER E. McCORQUODAI-E, Ja. 

